The steady-state operation of turbomachines, in particular gas turbines, is known in the prior art. In the endeavor to achieve a high efficiency, modern gas turbines, for example, with regard to their design, are constructed with the smallest possible radial gaps between the rotating elements and the elements arranged in a rotationally fixed position, for example between an inner wall of the flow passage of a gas turbine and moving blades arranged on a rotor of the gas turbine or between the rotor and guide blades arranged on the inner wall of the flow passage.
To set the radial gap, DE 44 11 616 shows a heating/cooling system for a rotor of a turbomachine. The temperature of the rotor is adapted during the transient operation of the turbomachine by virtue of the fact that a fluid affected by temperature can flow through the rotor via the activation of external valves.
Furthermore, U.S. Pat. No. 5,271,711 discloses rotor cooling for a gas turbine. Extending along the rotor is an annular space in which cooling air is directed from the compressor-side end of the rotor to the combustion-chamber-side end. At its outer boundary, the annular space has cooling-air openings in order to direct cooling air into the intermediate spaces formed by rotor disks. During the load operation of the gas turbine, the cooling air cools the intermediate spaces of the rotor disks.
The rotor-disk cooling of a gas turbine during the load operation has also been disclosed by DE 665 762.
In addition, DE 39 09 606 A1 discloses a rotor for an aircraft gas turbine through which a fluid flow can flow. In order to achieve a minimum loss of efficiency of the powerplant cycle, the rotor is either heated or cooled with the fluid during operation. During operation of the gas turbine, compressor inlet air is extracted from the compressor for cooling the rotor and is directed in the interior of the rotor along a hollow shaft and is discharged at the outlet of the turbine. To heat the rotor, compressed and thus already heated compressor air is extracted from the compressor downstream of the first compressor stage and is fed via an external valve to a mixing chamber in which the heated and compressed air mixes with the cool compressor inlet air. Hot air then flows through the cooler rotor.
In addition, U.S. Pat. No. 6,382,903 discloses the brief heating of the rotor of a gas turbine after it has been started in order to heat the rotor more quickly. To this end, the final compressor air heated by compression is fed to the rotor.
In addition, it is known that the limits for the reduction in the radial gaps are established by the hot-start problem. The hot-start problem occurs if, during the shutdown, that is to say during the cooling phase, of the gas turbine, the latter is restarted in the not yet completely cooled state. In the process, the rotor may jam in the flow passage, since the casing cools down more quickly than the rotor together with the moving-blade wheels and moving blades arranged on it. The still hot rotor cooling down more slowly cannot follow the thermally induced reductions in the dimensions of the casing. If the radial gaps are dimensioned to be too small, the blades lengthening on account of the centrifugal-force expansions will therefore jam the not yet completely cooled rotor after restarting of the gas turbine. This not only prevents the rotor from being able to rotate but also leads to costly damage to the gas turbine. Furthermore, the operating periods of the gas turbine are reduced, since the predetermined cooling phase must first be effected after the end of the firing of the gas turbine before the gas turbine can be safely restarted.
On the other hand, radial gaps which are dimensioned to be too large lead to an undesirably low efficiency of the gas turbine, since the process gas in the flow passage, in a manner not to be disregarded, flows through the radial gaps without transmitting at least some of its energy to the blades in accordance with the intended purpose.
This disadvantage becomes especially noticeable in gas turbines, since high temperatures prevail here in the flow passage.
But other turbomachines, such as compressors, steam turbines and the like, are also affected by this.
In particular the shutdown and the start-up of a gas turbine require a special procedure for design reasons, since the casing of the turbine and also the rotor shaft together with the elements arranged on it are adapted to thermal changes at a different speed. Special attention is therefore to be paid to the shutdown of a gas turbine on account of the problems explained above. This results in special requirements which must be heeded in order to avoid damage to the gas turbine; thus, inter alia, a minimum size of the radial gaps.
On the one hand, sufficiently large radial gaps are to be provided in order to avoid “constriction” of the rotor shaft with the elements arranged on it, this “constriction” leading to the jamming of the rotor in the casing. On the other hand, large radial gaps lead to the reduction, already mentioned, in the efficiency of the gas turbine. A further important aspect to be taken into account concerns the maintenance of the turbine, since maintenance cannot be carried out until after completion of the cooling phase of the gas turbine. As a rule, the period of time between the end of the firing of the gas turbine and the start of the maintenance work is about 24 hours. During the shutdown, the rotor shaft is continuously rotated at a reduced speed, for example at a speed of about 120 revolutions per minute, in order to shorten the cooling phase.